[mythtv-users] STAY AWAY FROM WI-FI !!!

Blammo blammo.doh at gmail.com
Thu Dec 8 15:22:13 EST 2005

On 12/8/05, Franco <mythtv at email.it> wrote:
> just wanted to tell everybody that now I have a working
> network at home so I can start experimenting with
> multiple frontend/backend configuration.

Without quoting everyone on this thread, let me throw in my 2cents.

Realized Throughput:
99% of consumer Wifi is half-duplex, meaning you need to take whatever
numbers being spit out, and whack them in half, right off the top, to
be safe. yes, I realize that you can burst up if your traffic isn't
symetric, but collisions still happen and the more your traffic is
2-way, the closer you are to half. Under heavy traffic, 40% is a
better number.

Using a tool like IPERF, you can get a pretty good idea of what your
setup will generate. I'm going to toss out some generalizations that
should be safe for most people, and assuming no other nodes / traffic

Keep in mind that 99% of wifi will shift speeds up/down as conditions
change. You may be at 11Mb until your dog walks over and plops down to
sleep on top of your base station.... :)

Most Wifi seems to perform better down to the endpoint, than back up.
Obviously, with two nodes on wifi, going through an access point, you
need to cut the numbers in half again. (MB is megabyte, Mb is megabit)
While some of these numbers may seem low to people, if down is 25Mb
and up is 15Mb, then 15Mb is our usable number.

802.11b: 4Mb
802.11g: 15Mb
802.11a: 20Mb
10Mb half-duplex: 4Mb
10Mb full-duplex: 8.5Mb
100Mb full-duplex: 85Mb
Gig full-duplex: 400-ish Mb (depends on card, interface, etc etc etc)

Data Rate / Bandwidth Usage

Within Myth you have to factor in the bandwidth usage of certain
activities to determine whether or not you can use wireless for
front/back connectivity.

-- non-HD mpeg2 (pvr-250 etc) playback/recording burns somewhere
between 5Mb and 7Mb depending on bitrate you choose for your
recordings. transcoding can change that as well. Over 5 minutes, I saw
an average of 5.68Mb.

802.11b = ok (depending on datarate and wifi rate, but see FFW/REW notes)
802.11g = ok
802.11a = ok
10M half-duplex = ok
10M full-duplex = ok
>=100M full-duplex = ok

-- HD mpeg2 (ts/ps) seems to burn between 10Mb and 30Mb. 5 minute
average I saw 14.52Mb watching a PBS HD 1080i show (Nature).

802.11b = insufficient bandwidth
802.11g = ok (depending on datarate and wifi rate, but see FFW/REW notes)
802.11a = ok (depending on datarate and wifi rate, but see FFW/REW notes)
10M half-duplex = insufficient bandwidth
10M full-duplex = insufficient bandwidth
>=100M full-duplex = ok

-- Fast Forward / Rewind burns BOATLOADS of bandwidth.  60x FFW on a
1080i recording gives between 50Mb and 80Mb (on a 100M segment)
averaging in the 60Mb area. Suprisingly 60x FFW on a non-HD recording
generates the same
amount of bandwidth. There is obviously some room for tuning here in the code.

-- Commercial Flagging, assuming you have enough horsepower, will
saturate the network segment as well, since the flagger is basicly
"watching" the program as fast as the CPU can process. I have a
machine that will generate 250Mb of bandwidth on gigE during the
"finding Logo" portion of flagging. Obviously a machine flagging
across wifi will saturate anything you have if the CPU is fast enough.

Given all of the above, you could use a non-HD frontend, that doesn't
commercial flag, as a valid node on your myth network. I'd personally
avoid it for anything else.

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